1. Field of the Invention
The present invention relates to a side-coupled standing-wave linear accelerator including a cascade of accelerating cavities linearly arranged along the axis of an energy beam and electromagnetically coupled by side cavities. More particularly, the present invention relates to such an acceleration of the variable-energy type including a non-resonant coupling side cavity switchable to electromagnetically couple and decouple a given pair of adjacent accelerating cavities so that the energy of a beam is discretely adjusted over a wide range while keeping a narrow spread of energy.
2. Descripiton of related art
So-called side-coupled standing-wave linear accelerators are used in association with X-ray tubes so that an accelerated electron beam is impinged on an X-ray radiation target. In such an X-ray generation, the following variable parameters of the standing-wave linear accelerator have generally been varied to control the energy of X-rays: an accelerating voltage of an electron beam, an input rf power, and an accelerating electron beam current.
Recently, another approach to energy control has been developed, which is to vary continuously the resonant mode patterns of the side coupling cavities, while keeping the constant resonance frequency. Such approaches are disclosed in U.S. Pat. No. 4,286,192 to Eiji Tanabe et al and U.S. Pat. No. 4,382,208 to Gard Meddaugh et al.
FIG. 1 is a schematic cross sectional view showing a pair of accelerating cavities 1 of a conventional side-coupled standing wave linear accelerator. The pair of accelerating cavities 1 are coupled by a drift tube 2 which allows passage of a beam of charged particles such an electrons, and also electromagnetically coupled by a "side" or "coupling" cavity 3, which is electromagnetically connected to each of the accelerating cavities 1 through an iris 4.
FIG. 2 is an equivalent circuit of the structure shown in FIG. 1. The left-hand cavity 1 is compared to a closed circuit composed of a capacitance C.sub.O, an inductance L.sub.O and a resistance R.sub.O in series. The equivalent circuit of the right-hand cavity 1 includes a capacitance C.sub.2, an inductance L.sub.2 and a resistance R.sub.2 connected in series. The equivalent circuit of coupling side cavity 3 includes a capacitance C.sub.1, an inductance L.sub.1, a resistance R.sub.1 and another inductance L.sub.1 all connected in series. One of the inductances L.sub.1 is coupled to the inductance L.sub.O with the coupling constant k.sub.01 and the other inductance L.sub.1 is coupled to the inductance L.sub.2 with the coupling constant k.sub.12.
If we designate the amplitude of the accelerating electric field of the left-hand accelerating cavity 1 as E.sub.0 and that of the right-hand one as E.sub.2, the ratio of the latter to the former is determined by the ratio of the electromagnetic coupling factor k.sub.01 between the left-hand accelerating cavity 1 and the coupling side cavity 3 to the electromagnetic coupling factor k.sub.12 between the right-hand accelerating cavity 1 and the coupling side cavity 4: E.sub.2 /E.sub.0 =-k.sub.01 /k.sub.12. Therefore, by varying the electromagnetic coupling ratio, the ratio of accelerating electric fields between adjacent accelerating cavities can be modified. The same effect can be obtained by introducing a difference of phase between coupled electromagnetic energies without varying the electromagnetic coupling ratio.
For this purpose, in the prior art, the difference in strength of accelerating electric field between the adjacent accelerating cavities has been adjusted or varied by changing the resonant mode of the coupling side cavity 3. The imporant matter is, in this case, to vary the degree of coupling while keeping the constant resonance frequency of the coupling side cavity 3.
The approaches just mentioned above, however, have the following drawbacks:
1. In the case of varying the accelerating voltage of the electron beam, the change of the voltage will cause the value of electron bunching to be inevitably separated from the optimum designated value of electron bunching, with the result that the spectrum of energy will spread.
2. In case of varying the input rf power or accelerating electron beam current, the energy spread increases as the modified value differs more from the initially optimized one, just as in the case above.
3. Energy spread as explained for the cases 1 and 2 leads not only to deterioration of output stability and reproducibility, but also to decrease in the current of the accelerated particles. As a consequence, the dosage rate by X-rays produced will decrease.
4. In case of continuously varying the degree of electromagnetic coupling or the phase difference between the accelerating cavities by changing continuously the resonance mode with a fixed resonance frequency, high precision is required for the manufacture and adjustment of a mechanical modulator used as a means of modifying the degree of coupling or the phase difference. In order to keep a constant resonant frequency and in order to make variable the degree of coupling, the position control with high precision of not greater than 0.2 mm and its high reproducibility are necessary. Such a modulator is difficult to fabricate, and furthermore, the reproducibility and stability fulfilling such a requirement cannot be obtained because of thermal change.